Intermittent fasting has been among the most talked-about diet ideas of the past decade. The pattern itself is simple: choose when you eat and when you do not. The most popular version, time-restricted eating, shrinks the daily eating window to 8 to 10 hours (or less) and leaves the remaining hours as a fast. Others, like the 5:2 pattern, restrict calories on two days a week. This calculator turns a protocol and a start time into a clear eating window and fasting period, with the 5:2 calorie targets used in the published trials.
Fasting is a way to structure meals, not a shortcut. What matters most is still how much you eat overall and how well you eat.
How to use the intermittent fasting calculator
- Choose a protocol. 14:10 and 16:8 are the common starting points.
- Enter the time of your first meal. The window starts then.
- Read the eating window and fasting period. If the window passes midnight, the clock wraps around.
- Check the 5:2 fast-day calories if you prefer a calorie-restricted-days pattern.
Fasting window formula: how the windows are calculated
Fasting starts when the window ends and lasts 24 − eating hours
Protocol N:M = N hours fasting, M hours eating (N + M = 24)
| Protocol | Fasting hours | Eating hours |
|---|---|---|
| 14:10 | 14 | 10 |
| 16:8 | 16 | 8 |
| 18:6 | 18 | 6 |
| 20:4 | 20 | 4 |
| OMAD | 23 | 1 |
The 5:2 fast-day targets (about 500 kcal for women and 600 kcal for men on two non-consecutive days) are those used in Harvie MN et al., The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women, International Journal of Obesity 2011;35:714–727.
Worked example
You choose 16:8 and your first meal at 12:00 PM.
- Eating window = 12:00 PM + 8 hours = 12:00 PM – 8:00 PM
- Fasting period = 8:00 PM – 12:00 PM, which is 16 hours.
With 20:4 starting at 7:30 PM the window is 7:30 PM – 11:30 PM and the fast runs until 7:30 PM the next day.
Windows for each protocol
| Protocol | Fasting | Eating window | If you start at 8:00 AM | If you start at 12:00 PM |
|---|---|---|---|---|
| 14:10 | 14 h | 10 h | 8:00 AM – 6:00 PM | 12:00 PM – 10:00 PM |
| 16:8 | 16 h | 8 h | 8:00 AM – 4:00 PM | 12:00 PM – 8:00 PM |
| 18:6 | 18 h | 6 h | 8:00 AM – 2:00 PM | 12:00 PM – 6:00 PM |
| 20:4 | 20 h | 4 h | 8:00 AM – 12:00 PM | 12:00 PM – 4:00 PM |
| OMAD (23:1) | 23 h | 1 h | 8:00 AM – 9:00 AM | 12:00 PM – 1:00 PM |
What the evidence says
- Weight loss comes mainly from eating less. A randomised trial of 16:8 eating in adults with obesity found no significant weight-loss advantage over eating normally (Lowe et al., JAMA Intern Med 2020;180:1491–1499).
- Early eating windows may help metabolism. A small controlled study of men with prediabetes found better insulin sensitivity with an early 6-hour window (Sutton et al., Cell Metab 2018;27:1212–1221), though the sample was only eight men.
- Long-term data are limited. A review in the New England Journal of Medicine concluded that more research is needed (de Cabo and Mattson, N Engl J Med 2019;381:2541–2551).
- Adherence is what counts. The best pattern is one you can keep.
Who should avoid fasting
Do not try prolonged fasting if you are pregnant or breastfeeding, under 18, underweight, or have had an eating disorder, and speak to your doctor first if you have diabetes, take regular medicines, have low blood pressure or any chronic illness. Stay hydrated (see the Water Intake Calculator), eat enough protein (Protein Intake Calculator) and plan your calories with the Calorie Calculator, TDEE Calculator and Calorie Deficit Calculator. The Macro Calculator helps design meals within your window.
This page is for information only and is not medical advice. Speak to a doctor about your own health, alcohol use, diet or exercise plans.
Sources
- Lowe DA, et al. Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Intern Med 2020;180:1491–1499.
- Sutton EF, et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab 2018;27:1212–1221.
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med 2019;381:2541–2551.
- Harvie MN, et al. Int J Obes 2011;35:714–727.
Frequently asked questions
What is intermittent fasting?
It is an eating pattern that cycles between periods of eating and fasting. Time-restricted eating (such as 16:8) limits food to a daily window; 5:2 involves two low-calorie days a week; alternate-day fasting alternates eating days and fasting days. All of them work mainly by reducing overall calorie intake.
Does intermittent fasting work for weight loss?
It can, mostly through eating fewer calories. A 12-week randomised trial of 16:8 time-restricted eating (Lowe et al., JAMA Internal Medicine 2020;180:1491–1499) found no significant difference in weight loss compared with eating three meals a day. A review in the New England Journal of Medicine (de Cabo and Mattson, 2019) concluded that fasting regimens have health effects but that more long-term human data are needed.
Is it safe for everyone?
No. Intermittent fasting is not advised for people who are pregnant or breastfeeding, under 18, underweight, or with a history of eating disorders, and people with diabetes who take insulin or other glucose-lowering drugs, or with other medical conditions, should only try it with medical advice.
What are the 5:2 fast-day calories?
In the studies by Harvie and colleagues (Int J Obes 2011;35:714–727) fast-day intake was about 500 kcal for women and 600 kcal for men, on two non-consecutive days, with normal eating on the other five.
What can I have during the fast?
Water, plain tea and black coffee are generally allowed in time-restricted eating. Staying hydrated matters; see the Water Intake Calculator. Avoid sugary drinks, which break the fast.